A pulsed plasma thruster is typically used to maneuver spacecraft and satellites in microgravity. The thruster employs a series of electric current pulses of limited duration and varying frequency between a pair of electrodes creating a series of electric arcs. The electric arcs pass over the surface of a propellant, increasing the surface temperature of the propellant, thereby forming an ionized gas, known as a plasma. The plasma is then exhausted from the device to produce thrust.
The two primary classifications of pulsed plasma thrusters are electrothermal and electromagnetic. In the case of an electrothermal thruster, the heating and/or ablation process results in a high chamber pressure accompanied by high plasma resistance, exhausting the plasma from the thruster by supersonic gas expansion. Alternatively, in the case of an electromagnetic thruster, chamber pressures and plasma resistance remain low, helping to facilitate high ionization fractions. Within an electromagnetic thruster, the flow of current between the electrodes induces electric and magnetic fields resulting in electromagnetic body forces which accelerate the ionized particles from the thruster. In comparison with electrothermal thrusters, electromagnetic thrusters are phenomenologically more complex. They are also considerably more difficult to model analytically and technologically more difficult to implement.
More specifically, electrothermal pulsed plasma thrusters may be characterized by high chamber pressures, a high plasma resistance, and substantial temperature and density gradients, typically followed with supersonic gas expansion through an exhausting insulating nozzle. The nozzle is generally insulating to reduce heat loss in the nozzle and encourage a modest amount of electromagnetic thrust. The high plasma resistance promotes efficient transfer of the capacitively stored energy into the plasma.
On the other hand, electromagnetic pulsed plasma thrusters may be characterized by low chamber pressures, a low plasma resistance, high ionization fractions, and pulsed electric arcs which traverse the region between the electrodes in a manner substantially perpendicular to the flow of the exhausting plasma. In the case of an electromagnetic pulsed plasma thruster, the addition of a supersonic nozzle usually provides little to no benefit since the low chamber pressures do not facilitate significant supersonic gas expansion. The primary contribution to the thrust is produced by the current's self-induced electromagnetic body forces. The present invention has found that a conductive nozzle, though ineffective at encouraging supersonic gas expansion due to low chamber pressures, aids in producing significantly greater electromagnetic body forces. Furthermore, a low plasma resistance promotes stronger electromagnetic body forces and in turn greater thrust.
Electrothermal and electromagnetic pulsed plasma thrusters may be further categorized as either parallel-plate or coaxial. In a parallel-plate configuration, the electric arc passes between a pair of electrodes that are situated parallel to the direction of the plasma flow as shown in FIG. 1. In a coaxial configuration, the electric arc passes between a centrally located electrode and an annular electrode as shown in FIG. 2. Generally, electromagnetic pulsed plasma thrusters utilize the parallel-plate configuration. In order to maximize electromagnetic body forces, the current path is necessarily substantially perpendicular to the flow of ionized particles. As FIG. 1 illustrates, the geometry of a parallel-plate design inherently provides the optimal current path relative to the flow of ionized particles. While significantly more difficult to achieve with a coaxial configuration, the present invention's geometry manages to encourage a current path substantially perpendicular to the flow of ionized particles in a coaxial configuration.
Prior art pulsed thruster systems can be found in U.S. Pat. Nos. 6,295,804 and 5,924,278 to common inventor Burton. Said systems utilize a high chamber pressure, accompanied by a high plasma resistance, to accelerate the plasma, thus classifying the systems as predominantly electrothermal. The systems accelerate the plasma through an insulating nozzle, facilitating supersonic gas expansion. The systems expel the plasma from the cavity in a flow path that is substantially parallel to the electric arc and current path within the cavity.
The present invention, predominantly electromagnetic, improves upon the prior art by maintaining a low plasma resistance and in turn producing strong electromagnetic body forces, resulting in significantly higher efficiencies and more consistent pulse-to-pulse performance. While the present invention utilizes a diverging nozzle, the nozzle is necessarily conducting, unlike to the aforementioned thrusters. Though the conducting nozzle enables some supersonic gas expansion, the major benefit is that the nozzle exploits electromagnetic phenomena to further accelerate the plasma. The electric arc and current path is also necessarily substantially perpendicular to the flow path of the plasma to facilitate electromagnetic acceleration.